Night vision goggles and head mounted displays require eyepieces or magnifiers that are color corrected for red, green and blue, with high image quality having modulation transfer functions (MTFs) greater than 50% at 40 line pairs per millimeter (lp/mm). These eyepieces or magnifiers cooperate with displays or screens of image tubes to bring images to the eyes of viewers. These eyepieces or magnifiers also require long eye relief greater than 20 mm, so that a viewer may have more tolerance in positioning his eyes to view an image.
A good eyepiece/magnifier must be insensitive to the position of the eye, because in a magnifier there is no exit-pupil-image of an objective lens to define the location of a user's eye. This requires a better level of correction, especially for spherical aberration so that the image does not swim or distort as the eye is moved around.
Examples of magnifiers disclosed in US patents are U.S. Pat. Nos. 2,885,928 and 2,900,871 issued in 1959 to James G. Baker; and U.S. Pat. Nos. 3,823,999 and 4,111,528 issued in 1974 and 1978, respectively, to Frits Johan Versteeg.
Eyepieces/magnifiers have been used to view a phosphor screen of a night vision image tube. These eyepieces, however, are difficult and expensive to manufacture. In addition, these image tubes required curved screens to aid in the correction of field curvature aberrations.
In the 1990s, eyepieces/magnifiers typically were configured similarly to the Qioptiq lens illustrated in FIG. 1. As shown, lens 10 includes three spherical glass lenses 13, 14 and 15; and one glass cemented doublet 11 and 12. Lens 10 provides good quality color image having an MTF greater than 50% at 40 lp/mm. Lens 10 receives an image from a curved surface of a phosphor screen, designated as 16, and transmits it to the eye of a viewer. Due to the number of lenses and their curvatures, lens 10 is difficult and costly to manufacture. Since lenses 13 and 14 are thin, they tend to fracture during manufacture, thereby resulting in low yields.
In the 2000s, an eyepiece was developed for use with both displays and screens, as shown in FIG. 2. As shown, lens 20 includes glass lens 21, plastic asphere lens 22 and plastic hybrid (diffractive) lens 23. Also included is a prism, which is comprised of two glass plates 24 and 25 for combining light projected from anode screen 26 and light projected from another display (not shown). Lens 20 introduces harmonics, however, that result in multiple images unacceptable to a viewer. The harmonics are due to steep angles of incidence at the diffractive lens.
In 2005, Edmund developed an eyepiece for use with both displays and screens, as shown in FIG. 3. As shown, lens 30 includes three glass lenses 32, 33 and 34 and plastic asphere lens 34. Also included is a prism, which is comprised of two glass plates 35 and 36 for combining light projected from anode screen 37 and light projected from another display (not shown). The addition of a fourth lens in lens 30 proved to be inadequate.
In 2007, Northrup Grumman designed an eyepiece illustrated in FIG. 4. As shown, lens 40 includes two glass doublets, positioned sequentially as first doublet 42 and 43, and second doublet 44 and 45. Also included are three glass lenses 41, 46 and 47. Thus, lens 40 is formed entirely from glass substances. However, lenses 46 and 47 include asphere surfaces for providing spherical correction. The lens 40 performs well but is expensive to produce. The lens has a wide field of view which is greater than 50 degrees. The lens, however, has no ability to combine an image projected from a display (not shown) with an image projected from screen 48, shown as rays of light 49 projecting towards the eye of the viewer (also indicated as eye pupil 49).
For fields of view that are less than 40 degrees, a glass doublet provides a cost effective eyepiece/magnifier having good image quality. As the field of view increases, or as a beam combiner is added, however, the eyepiece/magnifier becomes more complicated, resulting in a lens having five elements or more. In addition, the eyepiece/magnifier requires multiple aspheres for spherical correction. The eyepiece/magnifier, thus, results in a complex and expensive design, without good fusing capability.
The present invention, as will be explained, provides a cost effective eyepiece/magnifier that may combine an image projected from a display with an image formed directly on a screen/anode projected by an objective lens of a night vision goggle system. The combined images may be projected toward the viewer with a wide field of view. In addition, the eyepiece/magnifier may be formed from individual lenses having no flat surfaces, no asphere surfaces and no diffractive surfaces. Furthermore, the eyepiece/magnifier may project a wide field of view from a flat or curved screen surface.